Transportation vehicles typically include a suspension system that allows the wheels and tires of the vehicle to move relative to the body or frame of the vehicle. Springs are provided to support the weight of the vehicle and dampers are provided to dampen the motion of the suspension components. The damper typically takes a form of a strut or shock absorber and this strut or shock absorber is interconnected with the body or frame of the vehicle by a top mount. In the simplest designs, this top mount may be a solid structure. However, it is desirable that the top mount include some type of isolation arrangement to address NVH (noise, vibration, and harshness) issues and to assist in tuning of the vehicle suspension.
In some top mount designs, the upper part of the strut or shock absorber connects to an inner metal plate shaped like a large washer and this plate is positioned inside of a metal housing with the housing connected to the vehicle body or frame. A resilient material is provided between the plate and the housing to provide isolation. The resilient material is typically rubber though other materials are sometimes used. The resilient material acts as both a spring and a damper between the plate and the housing.
It is desirable that a top mount be very durable, lasting the entire life of the vehicle. At the same time, it is desirable that the spring rate and damping rate exhibited by the resilient material in the top mount remain within a specified range for as long as possible. Often, these design goals are in opposition. In order to achieve a desired combination of spring and damping rate in the isolation material, a very soft material may be used. However, this soft material may not provide the necessary durability.
It is known to use MCU (microcellular urethane) as a resilient material for a variety of applications. MCU is a urethane that has small “microcellular” bubbles or voids in the body of urethane material. The type of urethane and the quantity and size of the microcellular voids may be chosen so as to provide a desirable combination of spring rate and damping rate. However, MCU does not tolerate high loads. If a body of MCU is exposed to a compression load above a certain level, the MCU is irreparably damaged and the spring rate and/or damping rate is modified. This makes the use of MCU in suspension components such as top mounts difficult, since these components may see very high loads when the suspension experiences a large impact from rough roads or off-road use. If the MCU composition is chosen so as to provide the desired characteristics under normal suspension inputs, corresponding to roads that are smooth or have moderate irregularities, this composition may not tolerate the higher loads that the vehicle may periodically experience.